Inline extension into duckdb R package

[krlmlr]

Should we organize the development in a way that the relevant files from this repo can be copied into the duckdb-r repo and this extension becomes bundled with the R package?

Reasons:

• R (and perhaps even duckplyr) might be the only user ever for this extension
• duckplyr can look at the duckdb version to detect the capabilities of the extension (otherwise we'd need separate versioning for the extension)
• fewer things can go wrong with downloading and installing the extension

What are the downsides?

[romainfrancois]

Or does the extension belong in the duckdb-r repo so that it's always there ?

[krlmlr]

@hannes: Do you see downsides in including the rfuns extension into the R package? Who can help with the technical aspects of bundling the extension into the same shared library?

[krlmlr]

Hannes and I agreed to include it, for now, it's fine if the development happens in this repository.